Chemical reactions during submerged arc welding with FeO-MnO-SiO2 fluxes

Measurements have been made of the compositional changes of fluxes and weld metal during submerged arc welding with a series of synthetic MnO-FeO-SiO2 model fluxes containing 40 weight percentage SiO2 and different ratios of MnO to FeO. Mechanisms for the transfer of Mn, O, Si, C, S and P are discussed in terms of the thermodynamic driving forces and kinetic factors such as diffusion, nucleation, and segregation. One unique deduction is that interfacial, not bulk activities of components (such as FeO) govern transfer into (or out of) the metal phase. The concentrations of C, P, S and possibly Si tend to be larger in the molten weld metal than in the base plate.

Welding fluxes have been designed to satisfy a multiplicity of requirements such as arc stability, weld metal protection, weld metal deoxidation and alloying, slag detachability, etc. These requirements would be better fulfilled with further understanding of the many chemical reactions occuring simultaneously during welding. The purpose of this investigation is to initiate a systematic examination of the pyrometallurgical reactions and processes involved in welding steel. By performing experiments in which only a small number of parameters are varied in a well defined manner we hope to gain an understanding of the kinetic and thermodynamic factors which influence the composition of the weld metal and quality of the weld. Within this framework, we examine the thermodynamics and kinetics of the chemical and phase reactions which may be operative during welding in order to place limits on the range of possible reactions and processes which might occur in the complex range of materials, densities, temperatures, and states which lie in the relatively small distance between the filler metal feed and the metal to be welded.

Despite suggestions to the contrary, it is unlikely that equilibrium could be attained (except in small volumes) even at the high temperatures involved. This is so because of the very large temperature and density gradients, the presence of different phases-slag, metal, and plasma-the large electric currents, and the large radiative transfer of energy from the arc. Despite the expected departures from equilibrium, one may utilize equilibrium considerations, albeit cautiously, to place constraints on the chemical reactions and mechanisms involved in welding.

A common approach is to assume that a state of thermodynamic equilibrium is attained locally, on the basis that the high temperatures and high surface-to-volume ratio counteract the short time available for reactions to be completed. Chemical equilibrium between slag and metal during the welding process was initially suggested by Bojko. Measurements were later conducted methodically by Babcock and others. These investigators interpreted their results as a high-temperature equilibrium between slag and metal. An inspection of the mass action indexes derived from their data revealed that these indexes varied widely for the various consumables studied. Christensen and Chipman pointed out that no single slag parameter has been devised that will account accurately for observed variations of the equilibrium indexes whenever the slag composition is varied over a wide range. The amount of partitioning between the slag and the weld metal has also been found to be dependent on the welding parameters. This suggest that the pyrometallurgy is influenced by the processes that occur at the electrode tip, and the welding parameters affect the size of the metal droplets which in turn influences the chemical kinetics. However, when welding slag of similar compositions are compared. The mass action indexes do not have an apparent significance.

Submerged arc welding

Submerged arc welding is an automatic process. All the operator does is adjust the machine controls. A constant voltage power supply is employed, and current is controlled by the wire feed rate. The wire feeder and flux hopper ride on a stationary track. These arrangements make it possible to maintain constant arc length, slag depth, horizontal travel speed, electrode melting rate, voltage, current, etc.

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